The present invention relates to aircraft propulsion systems, and more particularly to control systems for aiding the prevention of the formation of contrails and/or for influencing the properties of contrails.
The term “contrail mitigation” is used herein to encompass the suppression of contrail formation and/or the alteration of contrail properties, for example to reduce the climate warming impact of contrails.
Contrails, also known as condensation trails or vapour trails, are line-shaped ice-clouds that appear behind aircraft under certain circumstances. The formation of a contrail depends on a number of factors, including: ambient temperature, humidity and pressure; the efficiency of the aircraft's engines; and the properties of the fuel burned in the engines.
A contrail, once formed, will typically dissipate within a minute or so, unless the ambient air is supersaturated with respect to ice, in which case the contrail may persist. A persistent contrail may grow over time to resemble natural cirrus cloud, both in size and optical properties, and is then referred to as “contrail-cirrus”. Line-shaped contrails and contrail-cirrus are collectively referred to as “aviation-induced cloudiness” (AIC). Contrail-cirrus is thought to cause a majority of the climate impact of AIC due to it being spatially-larger and longer-lived than non-persistent line-shaped contrails.
It has been found that contrail properties may be affected by the properties of the fuel that is burned in the engine of an aircraft. Kerosene and other hydrocarbon fuels typically contain a wide variety of types of molecule, characterised by their sizes (number of carbon atoms) and shapes. Some common molecule shapes include: chain-shaped molecules (paraffins), chains with branches (iso-paraffins), and chains wrapped into rings (cyclo-paraffins). Also common within many hydrocarbon fuels is the family of molecules known as “aromatics”, which are also ring-shaped but possess different properties from the cyclo-paraffins.
European Patent Applications 13168164.5 (published as EP2685280) and 13168163.7 (published as EP2677139), both owned by Rolls-Royce plc, disclose systems for altering contrail properties by mixing a plurality of fuel sources together in different proportions so as to vary the final fuel composition supplied to the engines. However it has been found that the delay, between instructing a final fuel composition and observing the resultant change in contrail properties, reduces the responsiveness of the iterative system to changing conditions.
Depending on the metric employed, the climate-warming impact of aviation-induced cloudiness may be of a similar magnitude to that of the CO2 emitted by aircraft, and may therefore represent a significant element of aviation's total climate impact. Contrail mitigation, and particularly the mitigation of persistent contrails, therefore represents a compelling opportunity for a significant reduction in the overall climate warming impact of aviation. However any additional operational penalties incurred in order to achieve contrail mitigation require careful scrutiny to determine whether the penalties outweigh the possible contrail mitigation benefits.
The time taken to determine and implement an appropriate level of actuation of a contrail mitigation scheme or mechanism can be significant to the effectiveness of any measures undertaken. For example an aircraft at cruise can cause contrail formation at a rate of typically 10 miles (16 km) per minute and so an effective contrail mitigation system must be able to implement an acceptable solution rapidly, in order to minimise the aircraft's contrail-related climate-change impact in a cost-effective manner.
It is therefore an object of the present invention to manage the mitigation of contrails in a manner that reduces negative impact on climate, for example while minimising the operational penalty incurred in doing so. It may be considered an additional or alternative aim to provide a system for aircraft engine contrail mitigation which can react quickly and/or effectively to changes in contrail-inducing conditions.